1. Field of the Invention
This invention relates to the field of propulsion systems and more particularly to the field of propulsion systems for torpedos and other underwater applications.
2. Description of the Prior Art
Recent torpedo propulsion system designs feature a closed Rankine cycle using purified water as the working fluid. These systems are commonly referred to as Stored Chemical Energy Propulsion systems (SCEPS). During the condensing portion of this cycle, heat is rejected to the ambient seawater by means of a conventional shell heat exchanger incorporated into the torpedo hull. The available volume and surface area of the largest condenser that can be packaged into a given vehicle dictates the maximum power of the propulsion system that can be incorporated in that vehicle.
FIG. 1 is a schematic diagram of a prior art torpedo propulsion system 10 based on the SCEPS totally closed concept. The thermodynamic cycle is based on a closed Rankine cycle scheme, similar to the MK 50 torpedo propulsion system, without regeneration for the feedwater. The energy required in the boiler 12 for steam generation is provided by the reaction heat of Lithium metallic fuel with gaseous sulfur hexafluoride (SF.sub.6) oxidizer. The fuel and the reaction products are retained within the boiler internal cavity during the life of the system. The liquid oxidant is carried on board in a separate tank 14 which is steam heated to produce the gaseous reactant for the boiler 12. The power required to operate the drive shaft 16 for propulsion, feedwater pump 18 and accessories such as hydraulic pump 20 and alternator and air pump 22, is provided by a steam turbine 24 via a gearbox 26. The heat is rejected from the cycle, as the exhaust steam from the turbine 24 is desuperheated, condensed and subcooled. These processes are performed in the passages of the condenser 28 integral with the torpedo/vehicle hull. Propulsion system performance is highly dependent on the condensing pressure level and heat transfer area and effectiveness of the surface condenser 28.
Presently, there is a desire to increase power, and hence speed, for existing torpedo configurations. For a given power system volume this dictates the need for higher power density systems than exist today.
A closed Rankine cycle system for the desired high power levels, may not be practical. Present SCEPS systems utilize condensers which are integral with the torpedo hulls. To satisfy these high power levels, the vehicle must have a greater hull surface area exposed to the sea than is available from the full length of the torpedo hulls. Consequently, the closed Rankine cycle propulsion system may not be able to provide sufficient power to satisfy future demands.
FIG. 2 shows an alternative prior art propulsion system. FIG. 2 is a schematic drawing of an open Rankine cycle system 50 operating with seawater working fluid. The components of this system are similar to the ones shown for the closed Rankine cycle 10 with the exception of the condenser 28 of the closed cycle 10. The spent steam from the turbine 52 of open cycle 50 is either directly exhausted into the ocean via a discharge line 54 or is bypassed to oxidant tank 56 to preheat the oxidant prior to being exhausted into the ocean via discharge line 58. Therefore, a condenser is not required. The water for the feedwater pump is taken directly from the surrounding ocean via water feed line 60. The ocean pressure provides the turbine's backpressure at discharge line 54. This has a considerable effect on the available energy and conversion efficiency of the turbine 52. As a consequence, as the torpedo operational depth increases and the corresponding backpressure at discharge line 54 increases, the cycle efficiency decreases dramatically.
In an open Rankine cycle power generation scheme, the heat rejection limitation of the closed Rankine cycle system does not exist because the ocean can be considered an unlimited heat sink. However, the thermodynamic efficiency of the open Rankine cycle system becomes very poor due to backpressure on the turbine exhaust as operating ambient pressure increases. To overcome the back pressure effects in meeting mission requirements, a prohibitively large supply of propellant must be provided on board the vehicle. Additionally, the increase in heat energy generation rate requires an increase of heat transfer surfaces, leading to a larger boiler component than may be needed with a good cycle efficiency.
Neither the closed Rankine cycle nor the open Rankine cycle propulsion system will likely meet desired higher power requirements at all operating depths. Consequently, there is a need for a propulsion system that combines the increased power producing capability of the open Rankine cycle and the efficiency of the closed Rankine cycle.